The fully developed remnant of a neutrino-driven supernova. Evolution of ejecta structure and asymmetries in SNR Cassiopeia A★

Abstract

Abridged. We aim at exploring to which extent the remnant keeps memory of the asymmetries that develop stochastically in the neutrino-heating layer due to hydrodynamic instabilities (e.g., convective overturn and the standing accretion shock instability) during the first second after core bounce. We coupled a 3D HD model of a neutrino-driven SN explosion with 3D MHD/HD simulations of the remnant formation. The simulations cover 2000 years of expansion and include all physical processes relevant to describe the complexities in the SN evolution and the subsequent interaction of the stellar debris with the wind of the progenitor star. The interaction of large-scale asymmetries left from the earliest phases of the explosion with the reverse shock produces, at the age of $\approx 350$~years, an ejecta structure and a remnant morphology which are remarkably similar to those observed in Cas A. Small-scale structures in the large-scale Fe-rich plumes created during the initial stages of the SN, combined with HD instabilities that develop after the passage of the reverse shock, naturally produce a pattern of ring- and crown-like structures of shocked ejecta. The consequence is a spatial inversion of the ejecta layers with Si-rich ejecta being physically interior to Fe-rich ejecta. The full-fledged remnant shows voids and cavities in the innermost unshocked ejecta resulting from the expansion of Fe-rich plumes and their inflation due to the decay of radioactive species. The asymmetric distributions of $^{44}$Ti and $^{56}$Fe and their abundance ratio are both compatible with those inferred from high-energy observations of Chandra and NuSTAR. The main asymmetries observed in the ejecta distribution of Cas A can be explained by the interaction of the reverse shock with the large-scale asymmetries that developed from stochastic processes that originate during the first seconds of the SN blast.